Oscillators constructed to operate in frequencies of microwave, millimeter wave and above are extremely small and it is extremely difficult to couple one or more active devices thereto. In fact, oscillators in the millimeter wave frequency range are so small it is difficult to couple a single active device, such as an IMPATT diode, thereto. Even for an oscillator operating in the microwave range it is difficult to couple a plurality of active devices to the oscillator to obtain higher output power levels. A typical prior art microwave oscillator including apparatus for utilizing a plurality of active devices to improve the power output, is illustrated and described in U.S. Pat. No. 4,034,314, entitled "Microwave Diode Coaxial Circuit Oscillator Improvement", issued July 5, 1977 and assigned to the same assignee. Because of the small size of the resonant cavities in the prior art devices, the number of active devices which may be utilized to increase the power output is severely limited.
Also in the prior art, an attempt was made to increase the size of the resonant cavity by operating the cavity in a higher order mode. In doing this, other undesirable resonances appear below and above the operating frequency. These resonances will generate spurious responses. To suppress these resonances this prior art structure utilizes absorptive filters to reduce the Q of the undesirable resonances. Unfortunately, this approach also reduces the Q of the desired resonance, resulting in an inefficient oscillator.
A related invention is disclosed in a co-pending application entitled "Microwave/Millimeter Wave Oscillator", filed Mar. 16, 1978, Ser. No. 887,142, and assigned to the same assignee. The apparatus described in this co-pending application is designed to solve the same problems as the present invention.